The invention relates to a device for measuring the speed of moving light-scattering objects by means of laser beams, with which a periodic intensity distribution is produced at the measuring point, and by means of a detecting device for light scattered by the object.
Such devices are known as laser anemometers. They serve especially to measure rates of flow such as in flowing fluids containing light-scattering particles. In the known technology, the periodic intensity distribution is produced in that two coherent component laser beams produce a virtual interference pattern at the measuring point. Because of the Doppler effect the pulse frequency for the light scattered by the particles traversing the measuring point is a function of speed. In the known technology, the frequencies can be evaluated using the detecting device for the light scattered by the object.
In order to produce a stable interference pattern at the measuring point, the light of the component beams must have a sufficient coherence length, so that presently, in practice, expensive lasers with a large space requirement have been used. It has already become known from German Offenlegungsschrift No. 34 35 423 to use laser diodes as well. In this case special measures have been employed to maintain a sufficient coherence length for the light emitted by the laser diodes so as to produce a stable interference pattern at the measuring point. In each case, the measurements depend greatly on external influences, which must be kept constant by means of appropriate controlling mechanisms.